This invention relates generally to the field of electromagnetic-based dermatological treatment systems, and more specifically to systems and methods for treatment of dermatological conditions with lasers having at least one wavelength determined by an optical parametric oscillator.
A variety of dermatological conditions are treatable using electromagnetic radiation (EMR). Sources of EMR for such treatments include lasers, flashlamps, and RF sources, each of which has distinct advantage and disadvantage profiles. EMR devices have been used, for example, treating abnormal pigmentation conditions, body sculpting (e.g., removal of subcutaneous adipose tissue), hair removal, treatment of vascular skin conditions (e.g., spider veins), reduction of wrinkles and fine lines, and dyschromia, among other conditions. Abnormal pigmentation conditions may include tattoos and benign pigmented lesions associated with high local concentrations of melanin in the skin, such as freckles, age spots, birthmarks, lentigines, and nevi, among other pigmentation conditions. Both pulsed and continuous-wave (CW) laser systems have been used to treat pigmentation conditions, although pulsed lasers are more frequently used.
Nanosecond lasers have been used for decades to treat pigmented lesions and tattoo removal. Nanosecond lasers, as used herein, are pulsed lasers having a pulse width (PW) or duration of greater than 1 nanosecond (nsec) up to 1 microsecond (μsec). By delivering the laser energy in a pulse with a very short time duration, highly localized heating (and destruction) of a tissue target structure (e.g., melanin, ink particles, collagen) can be achieved, thereby minimizing damage to non-target structures. Heating in tissues depends upon both the absorption coefficient of the irradiated tissue structures for the wavelength of laser light used, as well as their thermal relaxation times (TRT), which is a measure of how rapidly the affected structure returns to its original temperature. So long as the laser pulse duration is less than the thermal relaxation time of the target, no significant heat can escape into non-target structures, and damage to non-target structures is limited.
The availability of picosecond laser pulses has ushered in a new paradigm in tattoo removal. As used herein, picosecond lasers are pulsed lasers having a pulse width or duration of 1 picosecond (psec) up to (and preferably below) 1 nsec. Studies have shown that the diameter of tattoo ink particles can range from 35 nm to 200 nm, with clusters as large as 10 μm. To clear the tattoo ink, the particles must be broken up into smaller fragments that can be cleared by the body. To break the particles up effectively, the laser energy must be delivered within the TRT of the particle, since the energy that escapes into the surrounding tissue not only damages non-target structures but also is unavailable to break down the target structure. A simple dimensional analysis shows that the TRT of a spherical particle scales with the square of its diameter, and ink particles smaller than about 150 nm will have relaxation times below 1 ns.
While the pulse duration for nanosecond lasers is generally less than the TRT for melanin in the skin, the small size of many ink particles in tattoos can result in TRT times of less than 1 nanosecond for those particles. Consequently, the use of conventional Q-switched nanosecond lasers, which produce pulses of 5-20 nsec in duration, may result in ineffective ink removal as well as damage to tissue structures such as blood vessels, collagen, and melanin as the pulsed laser energy escapes into adjacent non-target tissue structures after the lapse of the TRT. This is particularly true for lasers having wavelengths that are highly absorbed by the non-target structures. Studies have shown that the use of picosecond lasers instead of nanosecond lasers can reduce the number of treatment sessions required to clear tattoos by a factor of 3.
Treatment of tattoos and pigmented lesions with picosecond laser pulses is a new and rapidly developing field in dermatology. Although nanosecond lasers are in theory should be adequate for removal of benign pigmented lesions because the relaxation time of melanin is greater than the pulse width for many nanosecond lasers, physicians have reported that lower treatment fluences are required when using picosecond laser pulses, which reduces thermal loading to tissue and the risk of adverse events. Thus, picosecond laser pulses may offer less tissue damage and higher safety margins for pigmented lesions, in addition to their superior performance for tattoo removal. The potential for improved clinical outcomes using picosecond lasers has resulted in commercially available systems having pulse widths of 500-1000 psec with pulse energies (i.e., energy per pulse) exceeding 100 mJ. On the other hand, high-energy picosecond lasers are much more complex and costly than any other energy-based treatment systems in the dermatology market today, and there is a need for more flexible, less expensive picosecond laser systems.
Tattoo removal presents a number of distinct challenges for laser-based pigmentation treatment systems. Tattoos are created by depositing thousands of ink particles below the epidermis into the dermis of the skin. The depth of ink particles may range from 250-750 μm, or more commonly 300-500 μm. In some instances, however, ink depths up to 1800 μm may occur. The wide particle size distribution, as already noted, also presents a challenge for effective tattoo removal while minimizing damage to surrounding structures.
Lasers remove tattoos by breaking down the ink particles that form the tattoo design with laser light at a wavelength that is highly absorbed by the ink used in the tattoo, and at a fluence (energy per area, typically expressed as J/cm3) sufficient to rupture the ink particles into smaller particles that can be removed by the body's natural repair systems.
Ink colors are determined based on their light absorption profile. A given color results from the ink absorbing complementary colors of light, i.e., colors opposite to the ink color on a color wheel. For example, because red and green are complementary colors, green inks appear green to the eye because they absorb colors in the red area of the visible light spectrum, while red inks appear red because they absorb colors in the green area of the visible light spectrum. Thus, green inks are more efficiently removed by red light, since green ink has a relatively high absorption of its complementary color. Conversely, red inks are best removed by green light because they highly absorb light in the green wavelengths.
Tattoos incorporating multiple ink colors present special challenges in laser-based removal systems, because multiple laser wavelengths may be necessary to remove all of the different ink colors. Thus, multiple laser sources may be used in some systems, resulting in systems that are much more expensive, complex, and bulky. To avoid damage to the skin because of the high energy fluences involved, many systems allow a user to vary the width of the laser beam applied to the tattoo.
Shading in tattoos presents another challenge to safe and efficient tattoo removal. Shading results in significant variations in the ink particle density (i.e., color intensity variation) across the tattoo area. Because of this, some systems allow a user to vary the pulse width (PW) of pulsed laser systems, also adding to the complexity of the system. In addition, because the ink particles may be located at different depths within the dermis, it is preferable for the laser light to have a high fluence even at relatively large beam diameters.
The first commercial dermatological picosecond laser systems used either a single 755 nm lasing wavelength, with alexandrite as the lasing medium, or dual 1064 nm and 532 nm laser wavelengths using Nd:YAG lasers. The 755 nm and 1064 nm wavelengths are part of the near-infrared portion of the electromagnetic spectrum, and are well-suited to removal of black tattoo inks due to their broad absorption spectra. The 532 nm wavelength is in the green portion of the visible spectrum, and is well-suited to removal of red inks which strongly absorb green light (the complementary color of red).
Because black and red are the most common tattoo colors, dual wavelength (532 nm and either 755 or 1064 nm) picosecond systems are the most common systems available. However, green and blue inks occur in about one-third of tattoos, and the absorption strength for these inks is greatest in the red portion of the visible spectrum. Accordingly, there is a need for a red wavelength in addition to the dual wavelength 1064/755, 532 nm (near infrared and green) picosecond laser systems to facilitate removal of green and blue inks. In view of the already-high cost of picosecond laser systems, the addition of a red wavelength must be done at a low cost, and in a flexible system that allows different wavelengths of light to be selected quickly and easily.
Because of their versatility, dual wavelength (1064/755, 532) picosecond systems are widely used to treat benign pigmented lesions, which involve the removal of melanin particles from the skin. Pulsed light at 532 nm is highly absorbed by melanin, while 1064 nm light absorbed less than 10% as well (absorption coefficients of 55.5 mm−1 and 4.9 mm−1) poorly absorbed. In addition, penetration depth of laser light falls rapidly with wavelength. Therefore, 532 nm laser light is effective at aggressive treatment of shallow pigment and 1064 nm light is more commonly used for milder but deeper treatment. It would be useful to have a third wavelength with an intermediate absorption in melanin.
Treatment of pigmented lesions can sometimes result in post-inflammatory hyperpigmentation or hypopigmentation. While the reason for such adverse events is uncertain, it is believed that this may result from injury of the laser light to blood vessels. Accordingly, effective wavelengths for treatment of pigments are those that minimize potential damage to blood vessels in the superficial dermis, and maximize the absorption of melanin relative to hemoglobin.
Pulsed red light has been provided in prior art laser systems, by laser-induced florescence of organic dyes. Typically, excitation is provided by a 532 nm (green light) Nd:YAG pulsed laser, with the red emission wavelength determined by the specific dye being used. Wavelengths of 585, 595, and 650 nm have been provided. Dyes are sometimes provided embedded in a sold substrate. In systems of this type, the minimum pulse duration is defined by the fluorescence lifetime of the dye, which is typically between 1-5 ns, precluding their use in picosecond laser systems. Incoherent (non-laser) light may be captured optically and focused onto a treatment plane.
In other systems, the dye cells may be used as the gain medium in a laser cavity to produce laser emission, in which case picosecond pulses are possible because the pulse duration is approximately equal to that of the excitation laser. However, the cost of assembling such systems is significantly increased relative to systems that do not require dyes, and becomes prohibitive if the dye cells must be replaced frequently.
A more fundamental limitation of dye systems is their susceptibility to optical degradation. Both output energy and beam profile uniformity fall rapidly with operation, typically within 10,000 laser shots or pulses. Fluence of the beam at the treatment plane therefore becomes irregular and continues to change over time, leading to poor clinical outcomes. Emission also tends to have low spatial coherence, making it difficult to deliver the beam through a fiber or articulated are to an applicator, such as a handpiece, for application to the patient.
Because of optical degradation issues, dye cells are typically designed as a consumable item that attaches to the end of the applicator (e.g., a handpiece). While this allows the user to change the dye cell when performance drops, restoring beam uniformity and fluence, it introduces several limitations. First, in multi-wavelength systems the dye cell must be removed to change wavelengths, which is inconvenient to the user and patient during removal of multi-colored tattoos requiring multiple wavelengths in a single treatment session. Second, because the dye cell is near the point of application, integration of photometry to detect the optical degradation is difficult because of space limitations. In spite of these limitations, dye cells have seen limited but consistent use in the field for decades because of their ability to provide multiple laser wavelengths.
Another known method for generating red-wavelength picosecond laser pulses is through second harmonic generation, in which the frequency of the pumping laser is doubled, resulting in an output having wavelengths that are half that of the pumping laser. For example, Nd:YAG lasing wavelengths such as 1319 or 1338 nm may be frequency doubled with nonlinear crystals to produce red picosecond pulses at 659 and 669 nm. However, pumping wavelengths capable of frequency doubling to provide red laser light have relatively low optical gain, making the cost and complexity at these wavelengths significantly greater than existing 1064 and 532 nm dual wavelength systems. In addition, wavelengths in the 1300 nm range have limited use for dermatology, and such systems would have only one wavelength of significant value unless more than one laser engine is provided in the system, which would significantly increase system complexity, cost and bulk. Such systems are not economical and have not been commercialized.
Finally, laser architectures outside of the red spectral region have been developed, but these systems sacrifice clinical efficacy because of the non-optimal wavelengths. For example, picosecond laser systems are available that produce 755 nm, near-infrared pulses using alexandrite as the lasing medium, as well as systems that using 532 nm picosecond pulses to pump a titanium sapphire oscillator,
There is a need for dermatological picosecond laser systems that are able to efficiently remove tattoos that incorporate a variety of ink colors, particle sizes and ink depths, and which are relatively compact, non-bulky and easy to use. There is also a need for dermatological picosecond laser systems having a simplified construction with fewer components, which are capable of providing a variety of laser wavelengths for treatment of a wide variety of pigmentation conditions and skin conditions, and allow a user to switch from a first to a second treatment wavelength quickly and easily.